Just play the strumming pattern over an Am chord and work on getting the accents down on the 1st and 3rd beats. Then, you can work with changing chords. Since we're playing 16th notes, we count using the syllables, "1-e-and-a-2-e-and-a..." etc. You can use a pick or just your fingers to strum. Try not to hit the 5th and 6th strings when you're strumming the D chord, with its root note on the 4th string!